Passive radar operates by gathering at a receiver radio frequency (RF) signals reflected from objects of interest and then processing the received signals to determine locations of these objects. A key characteristic of passive radar is the absence of a specific transmitted radar signal—the passive radar system instead relies on the (highly likely) presence of other RF transmitters in the locality, which themselves emit RF signals which the passive radar system can then employ. Such RF radiation that does exist will inevitably be reflected from an object of interest. A passive radar system relies on being able to detect them, and to correlate these reflections with signals received directly from the RF emitters. With a plurality of such correlations, triangulation can be used to determine the position of an object.
Passive radar is particularly useful in determining aircraft position, such as for the purposes of air traffic control. As it does not rely on the production of a specific radar emission, but instead processes signals resultant from reflections, by radio-reflective objects, of other RF emissions such as radio broadcast signals, communication signals or navigation signals, passive radar depends on the processing of a low power, relatively wide-band received RF signal.
A classical radar emits pulses and has a rotating directional antenna, which means that each target is viewed only intermittently. This means that it can be difficult to distinguish different types of target movement. In particular, it can be difficult to distinguish the patterns of the returns from aircraft and wind turbines.
Air traffic management (ATM) radar is particularly susceptible to disruption from the effects of wind turbines. Wind turbines are becoming increasingly prevalent. They are often relatively tall, within range of a low-flying aircraft. Turbine blades are constructed with structural strength and mass as the main considerations; RF reflectivity is generally not a major concern. Thus, it is entirely possible that a turbine blade will be a reflective object in the RF spectrum. The motion of a turbine blade can cause the generation of reflections of RF radiation, which may impinge on a passive radar receiver. As the blades of a wind turbine, in use, rotate about the turbine axis, the blades can cause momentary reflection of RF radiation directed at the passive radar receiver. At the receiver, these momentary reflections could, depending on the power of the RF impinging on the blade, cause periodic relatively high power “flashes” of RF energy. These flashes of RF emission can be difficult to distinguish from reflections from objects of interest. This can cause false tracks to be created which can distract an operator and can interfere with the tracking of genuine objects of interest. This phenomenon is amplified in the event that an object of interest (e.g. an aircraft) is travelling over a wind farm (i.e. a plurality of wind turbines concentrated in a designated area), wherein many different extraneous turbine-blade reflections may cause so many erroneous readings at a passive radar receiver that the position of an object of interest may be extremely difficult to discern.